Lord Bolingbroke, the eighteenth century political philosopher, said that "history is philosophy teaching by examples." Thucydides is reported to have said much the same thing two thousand years earlier.
I guess if I had to boil it down to a few words, I would say history is a larger way of looking at life.
There is still much work to be done, still much to learn.
The McCullough quote was found in American Thinker: Historical Illiteracy.